Stoughton Fire Capt. Bob O’Donnell ran across Boylston street, pulled himself over the security fence and saw the carnage – bodies, body parts, and blood. One person lay motionless. Another man, knocked down and wearing pants that were still smoldering, cried for help.

He ran across Boylston street, pulled himself over the security fence and saw the carnage – bodies, body parts, and blood.

So Stoughton Fire Capt. Bob O’Donnell did what he was trained to do – save lives.

One person lay motionless. Another man, knocked down and wearing pants that were still smoldering, cried for help. Dozens more languished in pain.

O’Donnell, an Easton resident and nurse, had come to the Boston Marathon to greet his 19-year-old marathoner son, Bob O’Donnell III, at the finish. About 2:50 p.m., he was standing on the bleachers, recording the race and preparing to capture his son’s triumphant arrival.

That’s when they came – two loud blasts, 10 seconds apart. Flame, smoke and chaos ensued. People screamed, and began running from the scene.

O’Donnell dropped his camera and ran toward the smoke, pushing through the crowd – and found a war zone.

He saw dozens of people missing limbs – severed arms, legs, and fingers lay atop blood-soaked concrete. His first mission – make tourniquets to stop the bleeding.

“It was awful. It was a sea of blood,” said O’Donnell, 56, pointing to the red stains of evidence left behind on his black sneakers on Tuesday. “You’re slipping in people’s blood, and they’re hanging onto each other.”

The explosions near the marathon’s finish line on Monday killed three people, wounded more than 170 and reawakened fears of terrorism.

Minutes after the blasts, O’Donnell said he became furious as he stood among the carnage and rushed from victim to victim to help. One of them was dead.

“My first gut instinct, I think, was immediate rage – that how dare someone do this to such an event, to so many people running for so many causes for nothing but good.”

O’Donnell has worked as a firefighter and as an emergency room nurse for more than three decades. He works at Milton Hospital, and has worked at Signature Healthcare Brockton Hospital. As he helped load victims into wheelchairs and into ambulances Monday, he feared secondary explosions.

He also feared that his son may have been injured.

“I knew my son wasn’t in front of the first (blast), but I didn’t have visibility to the second one,” O’Donnell said. “I was very fearful that he may have been caught.”

He cried as he recalled the tragic events while sitting alongside his son Tuesday, where he struggled to speak of the incident with a reporter.

“I still get all worked up when thinking about how close it was,” the fire captain said, tears billowing in his eyes as he hugged his son.

At first, Bob O’Donnell III didn’t know his father had rushed to the bombing scene to help the victims. He also didn’t know if anyone in his family, who were waiting for him at the finish line, had been injured.

Page 2 of 2 - Police stopped him and other runners as they approached the final mile of the 26.2-mile race.

“No one really knew what was going on,” the younger O’Donnell said, wearing a Boston Marathon medal around his neck. “It was gut-wrenching. It was nerve-wracking.”

As panic spread, the younger O’Donnell, who is also an emergency medical technician, said he tried to calm people down. He asked officers to help but police, who noticed he was limping after a 25-mile run, told him to first focus on walking.

An hour after the blasts, he used a friend’s cell phone to send a text through to his girlfriend. He later met up with his family at Fenway.

Along the way, he said, he saw the best in Bostonians. As he shivered in his running clothes, he said strangers offered him sweatshirts, food, water. A freshman at Saint Anselm College in New Hampshire, he is studying biology and plans to work as a physician’s assistant. He raised nearly $7,500 for Children’s Hospital in Boston before running the marathon.

It would be seven more hours until he saw his father.

They reunited in Easton just after 10 p.m. Monday, after two Stoughton firefighters, Acting Deputy Fire Chief Greg Goldberg and Firefighter Jeff Ledin, drove into Boston to bring O’Donnell home.

“We’re a tight family, and what happens to one happens to all of us,” the fire captain said of his colleagues.

O’Donnell said he has no regrets in rushing to help the victims.

“I saw all the good things that normal, ordinary citizens were doing to help the people,” he said, “and I knew that those same good people would be around my son if he needed it.”